The present invention relates generally to the field of electronics assembly, and more specifically to methods and apparatus for placing solder balls on conductive pads of semiconductor substrates.
During the manufacturing of semiconductor devices, it is common to place solder balls upon an array of conductive pads of a substrate of a semiconductor device and then reflow the solder balls in an oven to provide a series of electrical connections on the substrate. The array of solder balls is typically referred to as a ball grid array (BGA). In some applications, as many as 2000 solder balls about 0.012 inches to 0.030 inches in diameter are placed on a semiconductor substrate in an area of about four square inches. The large number of small sized solder balls used makes it difficult to consistently place a full array of solder balls on a semiconductor substrate using current machinery.
Currently, there are several different approaches for placing solder balls to form ball grid arrays on semiconductors and other electrical devices. In one approach, an array of solder balls is picked up with a vacuum head. Each solder ball is held by a separate vacuum nozzle in the vacuum head. The vacuum head then places the solder balls on the semiconductor substrate and releases the solder balls thereon.
In another approach, a mask is placed over the semiconductor substrate. The mask has an array of openings corresponding to the desired pattern of electrical connections on the substrate. A quantity of solder balls is then spread across the mask with an air knife or a squeegee causing solder balls to fall into the openings in the mask thereby positioning the solder balls in the desired pattern on the substrate.
In still another approach, a transfer substrate is formed with an array of indentations corresponding to the desired pattern of electrical connections on a semiconductor substrate. The array of indentations is then filled with solder balls, and the semiconductor substrate is brought face down into contact with the array of solder balls lying on the transfer substrate.
Using one of the above-described approaches, it is possible to place solder balls on a semiconductor substrate. However, these approaches typically have less than desired reliability and throughput, and therefore, it is desirable to provide a solder ball placement apparatus and method having improved reliability and throughput over the prior art approaches described above.
In one general aspect, the present invention features an apparatus for placing spheres at predetermined positions on a substrate. The apparatus includes a platform to support the substrate at a first sphere placement position in the apparatus, and a placement station disposed above the platform that places spheres at locations on the substrate. The placement station includes a first container having a chamber to contain spheres, and a first carrier tray having a substantially horizontal upper surface that forms a lower surface of the chamber. The upper surface has a first section and a second section. The second section has a plurality of holes formed therein to receive spheres. The first carrier tray is movable to position the second section between a fill position beneath the first container and a place position disposed over the first sphere placement position. The placement station is constructed and arranged to fill the plurality of holes with spheres when the second section is in the fill position, and place the spheres at the predetermined positions on the substrate when the second section is in the place position.
The placement station can further include a vision system positionable over the first carrier tray to view the spheres in the plurality of holes prior to placement of the spheres on the substrate. The vision system can be further positionable to view the spheres on the substrate after placement of the spheres on the substrate. At least a portion of the platform can be movable along a first axis and a second axis to position the substrate, wherein the first axis is orthogonal to the second axis, and wherein the first axis and the second axis define a plane that is substantially parallel to the upper surface of the first carrier tray. The apparatus can further include a first pedestal constructed and arranged to lift the substrate above the platform prior to placement of spheres on the substrate, the first pedestal being movable, independent of the platform, to align the predetermined positions on the substrate with spheres on the first carrier tray. The first pedestal can be constructed and arranged to provide movement of the substrate along the first axis and the second axis and to provide rotation of the substrate about an axis that is perpendicular to the plane defined by the first and the second axis. The vision system can be positionable over the pedestal to view the substrate prior to placement to determine any error in alignment between the predetermined positions on the substrate and spheres on the first carrier tray.
The placement station of the apparatus can further include a placement head. In one embodiment the placement head has a plurality of placement pins each corresponding to one of the plurality of holes in the upper surface of the first carrier tray, and the placement head is disposed above the first sphere placement position and can be lowered to place each of the pins in contact with a sphere in a hole of the first carrier tray to push the sphere through the hole and onto the substrate. In a second embodiment, the placement head may use pressurized air to push the spheres onto the substrate. The placement station can also further include a second container having a chamber to contain spheres, and a second carrier tray having a substantially horizontal upper surface that forms a lower surface of the chamber of the second container, the upper surface having a first section and a second section, the second section having a plurality of holes formed therein to receive spheres, the second carrier tray being movable to position the second section between a fill position beneath the second container and the place position. The placement station is constructed and arranged to fill the plurality of holes in the second carrier tray with spheres when the second section of the second carrier tray is in the fill position, and place the spheres at the predetermined positions on the substrate when the second section of the second carrier tray is in the place position.
The first carrier tray can include a lower section disposed beneath the second section of the upper surface, the lower section having a plurality of holes corresponding to the plurality of holes in the second section, the first carrier tray being constructed and arranged to allow relative movement between the lower section and the second section between a first relative position in which the holes in the second section are in align with the holes in the lower section and a second relative position in which the holes in the second section are not in align with the holes in the lower section.
The first container can include at least one deflection plate disposed within the chamber, the deflection plate having a deflection surface and being disposed in the chamber such that the deflection surface forms an obtuse angle with the upper surface of the first carrier tray. The apparatus can further include a conveyor system, disposed on the platform, to load and unload substrates from the apparatus, the conveyor system having a first conveyor section, a second conveyor section and a third conveyor section, each of the first conveyor section, the second conveyor section and the third conveyor section being independently controllable, the second conveyor section being disposed on the moveable portion of the platform.
The apparatus can further include a flux station, disposed above a flux position on the platform, that places flux on substrates. The flux station can further include a vision system positionable over the flux position to view a substrate at the flux position. The apparatus can further include a second pedestal constructed and arranged to lift a substrate at the flux position above the platform prior to placement of flux on the substrate, the second pedestal being movable, independent of the platform.
In a second general aspect, the invention features a method of placing a pattern of solder spheres at predetermined positions on a substrate using a carrier plate having holes formed in an upper surface thereof. The method includes steps of loading solder spheres into the carrier plate while the upper surface of the carrier plate is in a substantially horizontal position, positioning the substrate beneath the carrier plate, and pushing the solder spheres through the holes in the carrier plate and onto the substrate.
The method can include a step of using a vision system after the step of loading and before the step of pushing to verify that each of the holes contains a solder sphere. The method can further include a step of using the vision system to inspect the solder spheres on the substrate after the step of pushing. The step of positioning can include a step of aligning the substrate with the carrier plate using the vision system. The method can include a step of applying flux to the substrate. The upper surface of the carrier plate can includes a first substantially smooth section and a second section, the second section containing the holes, and the step of loading can include a step of moving the carrier plate beneath a container of solder balls from a first position to a second position, wherein in the first position, the first section of the upper surface of the carrier plate forms a bottom surface of the container, and in the second position the second portion of the carrier plate forms the bottom surface of the container. The step of positioning the substrate beneath the carrier plate can includes a step of moving the carrier tray from the second position to the first position. The carrier plate can be moved more quickly from the first position to the second position than from the second position to the first position to cause the solder spheres in the container to substantially fluidize when the carrier plate is moved from the first position to the second position. The step of pushing can include as step of applying pressurized air to the solder spheres.
In still another general aspect, the invention features an apparatus for placing a pattern of solder spheres at predetermined positions on a substrate. The apparatus includes means for loading solder spheres into holes in an upper surface of a the carrier plate while the upper surface of the carrier plate is in a substantially horizontal position, means for positioning the substrate beneath the carrier plate, and means for forcing the solder spheres from the holes in the carrier plate onto the substrate.
The apparatus can include means for verifying that each of the holes contains a solder sphere. The apparatus can further include means for inspecting the solder spheres on the substrate. The means for positioning can include means for aligning the substrate with the carrier plate using a vision system. The apparatus can further include means for applying flux to the substrate. The means for loading can include a container of solder balls, and means for substantially fluidizing the solder balls in the container. The means for forcing can include means for applying air to the solder spheres.
In still another general aspect, the invention features an apparatus for placing solder spheres at predetermined positions on a substrate. The apparatus includes a frame a platform, coupled to the frame, that supports the substrate at a sphere placement position in the apparatus, a carrier plate, coupled to the frame, having an upper surface with a plurality of holes formed therein to receive solder spheres, the carrier plate being movable between a fill position at which solder spheres are loaded into the holes and a place position at which the solder spheres are moved from the holes onto the predetermined positions on the substrate, and a vision system positionable over the carrier plate to view the spheres in the plurality of holes prior to placement of the spheres on the substrate.